"He didn't have anybody in the world," said Das, who spent a sleepless night after learning of the tragedy.

The hunt for the squat murder suspect sent police to local psychiatric wards and homeless shelters as investigators checked MetroCards purchases and chased other leads. "We're reasonably confident we will be able to identify the perpetrator," said NYPD Commissioner Raymond Kelly.

A sketch of the suspect was later released and a $12,000 reward announced.

Sen had no contact with the agitated woman responsible for his death before she suddenly pushed the unsuspecting victim as the 11-car train rumbled into the station, police said.

Medical Examiners remove Sunando Sen's body, after he was pushed to his death in front of an oncoming No. 7 train in Queens. (Danny Iudici for New York Daily News)

She was alternately mumbling, swearing and speaking to herself minutes earlier as Sen tried to stay warm by ducking into the station stairwell and peering out for the train.

The attacker “was sitting down as this man went to see if the train was approaching,” said NYPD spokesman Paul Browne. “She got up...and she came up behind him and pushed him onto the tracks.”

There was no time to stop the already-braking train or for five horrified eyewitnesses to rescue the doomed Sen. His body was found under the second car of the No. 7 train, dead from blunt impact injuries to the head.

Cops offered no motive for the seemingly random attack at the 40th St./Lowery St. stop in Sunnyside.

A police sketch of the woman suspected of pushing a man onto the subway tracks at the 40th Street/Lowery station in Sunnyside, Queens. (DCPI)

The suspect, after watching Sen's fall, made a beeline for the stairs. Police released a surveillance video showing her running down the street below the tracks.

The suspect was described as a heavy-set Hispanic woman, about 5-foot-5, in her 20s and wearing a blue, white and gray ski jacket. She wore a pair of gray-and-red Nike sneakers.

Nervous straphangers at the Queens subway station kept a wary eye on the platform during the Friday morning rush hour.

"A random person pushing you onto the tracks — it's every New Yorker's worst nightmare," said rider Mary Kang, 26, while waiting for a No. 7 to visit her mother.

"I know the chances are it won't happen to me, but I can't get it out of my mind," added Kang — who stood a full 7 feet from the platform's edge.

Isabella Reyes, 33, was spooked by the thought of two similar subway shove deaths in less than a month. A Queens man died on Dec. 3 in midtown when a drifter shoved him in front of an oncoming Q train.

"One time is a fluke," said the Queens resident. "Twice is a pattern. I don't want to be the next victim."